1. Field of the Invention
The present invention concerns a method and a device for determining the homogeneity of magnetic dispersions, especially the distribution of the magnetizable powder in the dispersion, and the presence of agglomerates.
In the production of magnetic tape for recording information signals, a magnetic dispersion is applied in known manner to a nonmagnetic base usually a plastics film.
Such a magnetic dispersion consists essentially of a binder dissolved in a solvent and of magnetic particles having a size of from 0.1 to 2.0 .mu.m finely dispersed therein, this binder ensuring good adhesion of the magnetic particles to the film after the solvent has been evaporated by drying the coated web.
Inhomogeneities, i.e. fluctuations in the density of the magnetizable powder in the dispersion, and agglomerates contained in the dispersion, not only produce fluctuations in density in the finished dry magnetic coating, but also an uneven surface, due to the fact that regions of varying density shrink to different degrees during drying. Density variations in the magnetic layer lead directly, and surface roughness, which causes the distance between the magnetic tape and head during recording to vary, leads indirectly, to fluctuations in the remanent flux in the recording medium. An uneven surface, moreover, generally causes recording and scanning losses.
Very high demands are placed upon the uniformity of the remanent magnetic flux in a tape, these demands depending on the intended application of the tape. The recording of audio signals, for example, is particularly critical in this respect, because the human ear can perceive amplitude fluctuations of approx. 3%. In the recording of digital information, larger fluctuations are admissible, but dropouts are undesirable. In video recording where signals of very short wavelength are stored, scanning losses should be kept to a minimum by keeping the roughness of the tape surface very low. The number of dropouts, caused essentially by variations in the distance between the head and tape, should be kept to a minimum.
2. Description of the Prior Art
A high degree of homogeneity in the magnetic dispersion is therefore decisive for the quality of magnetic tape produced therewith. Up to now, it has not been possible to test magnetic dispersions directly for their homogeneity.
A trial coating always had to be made, on the basis of which it was then decided to terminate or continue processing of the magnetic dispersion, e.g. by extending the grinding period, increasing the grinding intensity of carrying out further filtrations of the made-up magnetic dispersion. The economic production of a dispersion and optimum dispersion were therefore not possible.